Sahih Muslim , Book 6, Hadith 2401 (USC-MSA) ⓘ

Ibn 'Umar (Allah be pleased with both of them) reported that the Messenger of Allah had two Mu'adhdhins, Bilal and son of Umm Maktum, the blind. The Messenger of Allah said: Bilal announces Adhan at (the fag end of the) night (i. e. Sahri), so eat and drink till the son of Umm Maktum announces Adhan. And he (the narrator) said: And the (difference of time) between their (Adhans) was not more than this that one climbed down (from the minaret) and the other climbed up (to announce Adhan).

حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، - رضى الله عنهما - قَالَ كَانَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم مُؤَذِّنَانِ بِلاَلٌ وَابْنُ أُمِّ مَكْتُومٍ الأَعْمَى فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ بِلاَلاً يُؤَذِّنُ بِلَيْلٍ فَكُلُوا وَاشْرَبُوا حَتَّى يُؤَذِّنَ ابْنُ أُمِّ مَكْتُومٍ ‏" ‏ ‏.‏ قَالَ وَلَمْ يَكُنْ بَيْنَهُمَا إِلاَّ أَنْ يَنْزِلَ هَذَا وَيَرْقَى هَذَا ‏.‏